The present invention relates to collector arrangements for electron beam tubes. In particular, the invention relates to arrangements for efficiently collecting electrons whilst avoiding negative effects due to secondary emission of electrons.
Electron beam tubes are used for the amplification of RF signals and are typically linear beam devices. There are various types of linear electron beam tube known to those skilled in the art, two examples of which are the klystron and the Inductive Output Tube (IOT). Linear electron beam tubes incorporate an electron gun for the generation of an electron beam of an appropriate power. The electron gun includes a cathode heated to a high temperature so that the application of an electric field between the cathode and an anode results in the emission of electrons. Typically, the anode is held at ground potential and the cathode at a large negative potential of the order of tens of kilovolts.
Electron beam tubes used as amplifiers broadly comprise three sections. An electron gun generates an electron beam, which is modulated by application of an input signal. The electron beam then passes into a second section known as the interaction region, which is surrounded by a cavity arrangement including an output cavity arrangement from which the amplified signal is extracted. The third stage is a collector, which collects the spent electron beam.
In an inductive output tube (IOT) a grid is placed close to and in front of the cathode, and the RF signal to be amplified is applied between the cathode and the grid so that the electron beam generated in the gun is density modulated. The density modulated electron beam is directed through an RF interaction region, which includes one or more resonant cavities, including an output cavity arrangement. The beam may be focused by a magnetic means to ensure that it passes through the RF region and delivers power at an output section within the interaction region where the amplified RF signal is extracted. After passing through the output section, the beam enters the collector where it is collected and the remaining power is dissipated. The amount of power which needs to be dissipated depends upon the efficiency of the linear beam tube, this being the difference between the power of the beam generated at the electron gun region and the RF power extracted in the output coupling of the RF region.
The difference between an IOT and a Klystron is that in an IOT, the RF input signal is applied between a cathode and a grid close to the front of the cathode. This causes density modulation of the electron beam. In contrast, a klystron velocity modulates an electron beam, which then enters a drift space in which electrons that have been speeded up catch up with electrons that have been slowed down. The bunches are thus formed in the drift space, rather than in the gun region itself.
In IOT, Klystrons and other linear beam tube types such as TWTs, the efficiency of collection of the electron beam can be improved by using a multi-stage depressed collector. In such an arrangement, there is a plurality of electrically isolated stages or electrodes, each operating at a potential at or between ground and the cathode potential. In one such typical arrangement, a collector has five stages, the difference in potential between the various stages being 25% of the beam voltage. By using such a multi-stage depressed collector, the electrons in the beam are slowed down before impacting on the electrode surfaces, thus leading to greater recovery of energy. Collectors may, of course, have a different number of stages operating at different potentials. The term “depressed” is used in the sense that the voltage at which each electrode is held is “depressed” in relation to ground potential.
We have appreciated the need to improve the efficiency of electron collection in an electron beam tube.